It is known that high speed turboprops offer significantly higher propulsion efficiency compared with that of the high-bypass-ratio turbofan engines. However, before the potential fuel savings associated with the advanced turboprops can be realized, several important tenchological problems must be solved, one of the more serious problems being excessive aircraft cabin noise.
It is widely recognized that the cabin or interior noise of propeller-driven aircraft is significantly more annoying than the noise environment inside a jet-powered aircraft. An effective control of interior noise for large turboprop aircraft is a major obstacle in aeroacoustics, mainly because of the high level of low frequency discrete tones generated by the turboprops.
Several solutions for the control of air-borne or structure-borne interior noise of the turboprop aircraft are currently being pursued. These solutions range from the conventional approach of developing improved fuselage soundproofing methods to more sophisticated techniques such as the use of synchrophased propellers to minimize the near-field noise signature impinging on selected parts of the fuselage structure. However, each proposed solution encompasses certain definite and distinct disadvantages.
Noise reduction, utilizing the installation of soundproofing materials as part of the fuselage, has not been found to be very effective for low-frequency noise. Furthermore, passive soundproofing methods incur significant cost and weight problems, an onerous situation as applied to aircraft.
The propeller synchrophasing technique requires an accurate automatic control of the propellers so that a predetermined phase relationship between the circumferential blade locations of the propellers is maintained. Although the synchrophasing technique has been successful in reducing the annoying variations in noise level at low frequencies, its effectiveness has been limited by electronics and mechanical control problems. Further, the surface area of the fuselage that can be effectively protected is somewhat constrained.
In view of the drawbacks associated with the above-noted methods, an alternative approach, commonly referred to as active noise control, is proposed by the present inventors for reducing the interior cabin noise of a propeller/propfan aircraft. The basic principle of active noise control is to reduce the noise radiated from a primary source utilizing a secondary sound source. Since the secondary sound source signal is made identical in amplitude but opposite in phase to the primary sound source signal, a complete cancellation can be achieved within certain regions of the space surrounding the two sources.
Although known, the practical implementation of active noise control is somewhat limited. To data, the most successful utilization of this technique has been restricted to noise attenuation inside of fluid ducts. While active noise control has been applied to attenuate a near-field environment, such as a hallway or chamber, this technique has not been directed at reducing the interior cabin noise for a propeller driven aircraft as proposed by the present invention.